Li Jun spent so much time fighting Yin Gen, even giving the latter so much time to remove the influence of his Duyi Realm because he had a purpose–he wanted to study his use of Willpower Manifestation so he could reach the same realm.
'I've learned a lot, but it's not enough,' he concluded. Willpower Manifestation is not as simple as the name suggested. Li Jun has read about some other World Communities' cultivation systems that train Willpower to affect the world or even control the Law or Dao.
But this is not the same as the body refiner's Willpower Manifestation. These methods are, at best, a way to train to reach such a stage. The word "manifestation" represented a sublimation of their Willpower to a higher form.
This sublimation allowed body refiners to use their Willpower the same way as Dao Will–including using it to remove Emperor's Dao Imprint in the Grand Dao Source.